Propene (C3H6), often also referred to as propylene, is one of the most important starting substances of the chemical industry. The demand for the base material propylene is increasing worldwide, wherein propylene just like ethylene mostly is produced from petroleum in a steam cracker in a ratio dependent on the process conditions and the raw materials.
To obtain additional propylene, a number of processes exist, such as the PDH process which proceeds from propane as educt. What is known, however, above all is the so-called MTP process, in which olefins are produced from methanol (MeOH) or dimethyl ether (DME) by catalytic conversion on a zeolitic catalyst. By varying the catalyst and the process conditions, the selectivity of the products obtained can be influenced and the product spectrum thus can be shifted towards short-chain olefins (then often also the process name Methanol-to-Olefin (MTO)), towards longer-chain products (then often also the process name Methanol-to-Gasoline (MTG)) or towards propylene.
The fundamentals of an MTP process are described for example in DE 10 2005 048 931 A1. From an educt mixture containing steam and oxygenates such as methanol and/or dimethyl ether, C2 to C4 olefins are produced above all. By a heterogeneously catalyzed reaction in at least one reactor, the educt mixture is converted to a reaction mixture comprising low-molecular olefins and gasoline hydrocarbons. By a suitable separation concept, higher olefins, above all the C5+ fraction, can at least partly be recirculated into the reactor as recycling stream and in said reactor for the most part be converted to propylene, whereby the propylene yield is increased.
Due to the multitude of components obtained in the product spectrum, in particular the multitude of olefins obtained, the purification system of an MTP process mostly is very complex.
From US 2004/0122272 A1 it is known that in a first separating means the C3− fraction, an aqueous fraction as well as at least one C4− and C5 fraction already are separated from each other. The C3− fraction then is processed further.
From U.S. Pat. No. 7,855,312 B2 it is known that first the oxygenates contained in the product stream are washed out via a water wash, before the individual carbon fractions are separated from each other.
U.S. Pat. No. 7,919,660 B2 describes how non-converted oxygenates, above all dimethyl ether, can be separated from light olefins, i.e. olefins with a small chain length, by means of a water wash.
According to WO 2010/104579 the light olefins obtained by the treatment are subjected to a water wash, wherein this wash can be effected at different points of the process.
WO 2006/048098 describes how a liquid fraction of hydrocarbons and oxygenates is charged to a first column in which the low boilers are separated from the high boilers. The separation of aqueous streams is not discussed.
From US 2003/0125597 A1 it is known to split up the olefin stream into a C4+ fraction and a C4− fraction after cleaning the olefin stream from aqueous components. US 2012/0083634 A1 likewise teaches a process in which a light and a heavy stream rich in hydrocarbons is produced, wherein here a C2 stream and a C3+ stream are obtained. Here as well, the separation of the hydrocarbon streams is effected only after the removal of water.
From US 2008/0242908 A1 it is known that first the water can be removed and the remaining organic stream is separated into a C3− fraction and a C4+ fraction and a third stream containing dimethyl ether.
US 2004/0267068 A1, US 2004/0267069 A1 and US 2004/0267077 A1 all discuss a similar separation concept, in which after passing a quenching system the hydrocarbon stream is subjected to a wash for removing the oxygenates, without previously or in this step a separation of the hydrocarbons being effected in dependence on their chain length.
US 2006/0135632 A1 describes the separation of the entire olefin stream from an aqueous fraction containing the oxygenates, without a detailed discussion of the further treatment of the olefins.
From U.S. Pat. No. 6,844,470 B2 a process is known, in which the C4+ fraction is separated from the oxygenates in a first separating operation, and in a second separating operation the C3− fraction then is separated from the dimethyl ether in essence and the extracting agent used in this separating operation. In the first separating operation there can also be obtained a third stream containing the C2− fraction. In each of the two variants, however, contained water must already be separated previously.
US 2004/0127758 A1 finally describes a process for the purification of the mixture obtained from an MTP process, in which after a separate separation of the water, the stream rich in carbon is separated into a C3− fraction and a fraction containing C4+ and dimethyl ether.